Chemical and thermal homogeneity in glass is a crucial part of good forming operations. The function of a glass melter is generally to produce glass with acceptable levels of gaseous or solid inclusions, but this glass usually has cord (or striae or ream) of chemically dissimilar phases. These non-homogeneous components of the parent glass result from a variety of normal occurrences during the melting process including refractory dissolution, melting stratification, glass surface volatilization, and temperature differences. The resulting cords are visible in the parent glass because of color and/or index differences.
One approach for improving the homogeneity of glass is to pass the molten glass through a vertically-oriented stir chamber located downstream of the melter. Such stir chambers are equipped with a stirrer having a central shaft which is rotated by a suitable motor. A plurality of blades extend from the shaft and serve to mix the molten glass as it passes from the top to the bottom of the stir chamber. The present invention is concerned with the operation of such stir chambers and, in particular, with achieving high throughput and high mixing efficiency (mixing effectiveness) from such a chamber without introducing defects into the resulting glass, specifically, defects arising from the erosion of the wall of the stir chamber and/or the surfaces of the stirrer as a result of the mixing process.
A simple way of picturing what a stirrer does under laminar flow conditions is to think of the cord as lumps of off-composition glass surrounded by glass of desired, or parent, composition. Each piece of cord can be thought of as having an interface between it and the parent glass. A measure of the total inhomogeneity of the glass is the total interfacial surface area of the cord. The minimum interfacial surface area occurs when all of the cord is in one spherical lump. As the lumps are broken into smaller parts and are stretched out into flat planes, the interfacial surface area is increased despite the fact that the volume of cord remains the same. A measure of the efficiency of stirring (also referred to herein as the effectiveness of stirring) is the ratio of the increased interfacial area after stirring to that before stirring.
To be effective in increasing homogeneity, a stirring system should perform the following three functions:
(1) It should stretch the individual lumps of inhomogeneous glass into thin streaks. This function requires the application of shear stress to the glass.
(2) It should cut the streaks into short segments. This function can be achieved through flow of the molten glass in a direction normal to the plane of the stirrer's blades.
(3) It should disperse the short segments such that there is no recognizable pattern. This function can be achieved through the selection of blade shapes that push glass normal to the direction of bulk flow, i.e., blade shapes that produce at least some radial flow of the glass.
Making the streaks thin and cutting them makes them individually difficult to see on a microscopic scale. Dispersing them eliminates the possibility that a visible pattern will be left on a macroscopic scale.
In a process where the flow of glass is continuous, these three functions must take place in a discrete time interval determined by the residence time of the glass in the stir chamber. As the flow rate of glass is increased, the glass has less time in the chamber for these three functions to take place. The usual engineering response to a desired increase in flow is an increase in stirrer speed. This increases the shear stress, the cutting frequency, and potentially also the dispersion rate.
Traditionally, glass stirring systems have been designed to have the highest shear stress possible consistent with reasonable stirrer life. Indeed, such systems are normally designed to produce high shear stress even when operated at low speeds. The intent is to get the most stirring from the smallest stirring system because of the high cost of the precious metals (e.g., platinum alloys) from which the stirring system is fabricated. In general terms, shear stress is increased by increasing blade speed and/or reducing the clearance between the stirrer's blades and the wall of the stir chamber.
For many glass products (e.g., architectural glass), only moderate homogeneity requirements apply. However, other glass products must meet stringent homogeneity and other quality standards. LCD glass is in this latter category. For this glass, both cord and inclusions need to be minimized and/or eliminated.
In accordance with the invention, it was discovered that in the process of making LCD glass, precious metal inclusions (e.g., platinum alloy inclusions) having a size less than 50 microns were being introduced into the LCD glass during its manufacture. These inclusions were traced to the stir chamber and, in particular, to erosion of the stirrer and the stir chamber wall as a result of viscous shear stresses created by the motion of the stirrer through the viscous molten glass.
One of the objects of the present invention is thus to minimize the creation of precious metal inclusions during the stirring of molten glass. However, this primary object is supplemented by the objects of: (1) maintaining high glass throughput, and (2) maintaining high stirring efficiency (e.g., low levels of cord). These latter objects and the primary object pull in opposite directions, e.g., one can reduce stirrer speed to reduce sheer stress and thus erosion, but reduced stirrer speed means less efficient stirring and/or reduced throughput.
As discussed below, the present invention is able to simultaneously achieve these seemingly contradictory goals by means of relationships between stirrer speed, stirrer/stir chamber geometry, and glass viscosity which allow sheer stress to be reduced below the level where unacceptable levels of inclusions are formed (e.g., the sheer stress acting on the stirrer and the stir chamber wall can be made less than 3.5×10−3 N/m2) while at the same time stirring efficiency and throughput are maintained at levels previously only achieved with high sheer stirring.